BSS guidelines here and Soliftec guidelines here both say the flue should be insulated. They are only guidelines, though, which is why I said ‘supposed to’. Certainly when I needed a new flue on my stove, the boat yard would only let it be a single wall one because they were replacing like with like. I’m not sure anyone is ever likely to check (or even know, probably), but if you want to abide by the latest guidelines, then it’s pretty clear what flue you should use (note, should, not must, because they are only guidelines)